Combing Through the Science of Hair Loss

While it may start with finding more hair in your brush, some thinning around your hairline, or noticing some sparse patches along the crown, the visible effects of hair loss can quickly add up. These changes can sometimes be gradual, but the impact can still feel significant, and it’s okay to feel frustrated or even a little anxious.The good news is that understanding the underlying cause of your hair loss—hormonal changes, genetics, or other factors—can empower you to take action and help restore your confidence.

The Anatomy of a Hair Follicle

One of the reasons hair loss can be so elusive is due to the complex nature of the hair follicle. Understanding its anatomy is essential for recognizing how hair growth occurs and why various factors can impact this process.The follicle consists of several interconnected parts, each contributing to the formation and maintenance of healthy hair:

• Hair Bulb: Located at the follicle's base, the hair bulb is where new hair cells are created. This part contains the dermal papilla, a specialized area of cells and blood vessels that provides nutrients and growth signals to help hair grow healthy and strong.[1] Notably, the thickness of your hair is correlated to the dermal papilla's volume and the number of dermal papilla cells (DPCs). When these cells get depleted, it prevents the follicle from regenerating hair, resulting in the cessation of growth.[2]
• Matrix Cells: The matrix cells in the bulb are responsible for creating new hair. These stem cells divide rapidly and harden over time, forming the internal structures of the hair shaft.[1]
• Sebaceous Glands: These glands are embedded in the hair follicle and produce sebum, an oily substance that helps protect your hair and skin. This prevents your hair from getting damaged and becoming dry and brittle.[1]
• Arrector Pili Muscle: This tiny muscle is responsible for causing your hair to stand up when you get goosebumps in response to the cold or stress.[1]

The Phases of Hair Growth

Hair growth isn't a constant process—it happens in cycles, meaning that your hair constantly moves through different phases at any given time. Shedding hair is an entirely natural process that can occur at any age, as individual hairs typically go through this cycle at different rates. Here’s how your hair follicles transition through this cycle:

• Anagen Phase (Growing): The anagen phase is the growth period of your hair. This is when the hair actively grows from the follicle, with your hair's cells dividing and pushing the hair shaft upwards. This phase can last anywhere from two to seven years, depending on factors like your genetics, health, and even your age. The longer your anagen phase lasts, the longer your hair can grow—usually around half an inch per month.[3]
• Catagen Phase (Transitioning): The catagen phase lasts only about 2-3 weeks, but it's essential to hair health. Even the most well-cared-for hair accumulates damage with times like split ends, fraying, or brittleness. Cells in damaged hair follicles are signaled to slow growth and undergo apoptosis to initiate a new cycle and promote new hair growth. During this time, the follicle detaches from the blood supply and condenses into a bulb of keratin called a club root.[4]
• Telogen Phase(Shedding): The telogen phase is the resting period for hair. In this stage, hair isn’t growing but stays in the follicle until the new hair pushes it out of the scalp. On average, about 10-15% of your hair is in this phase at any time. The phase lasts 3-4 months, and it’s completely normal to shed 50-100 hairs a day.[4,5]
• Kenogen Phase (Lag): Kenogen is a rarer and less often recognized phase between hair shedding (exogen) and re-entering the anagen phase. Not all hair follicles experience this lag in re-growth, but it is highly associated with pattern hair loss, stress, or illness.[6]

What Are The Types of Hair Loss?

When any part of the delicate systems within your hair follicles gets disrupted, it can lead to changes in the health and appearance of your hair. Here are some of the most common forms of hair loss associated with changes in hair health.

Pattern Hair Loss

Maybe it’s a receding hairline, a loss of volume, or some bald spots showing up; pattern hair loss is easy to spot. Also known as male-pattern baldness or female-pattern hair loss, pattern hair loss is the most common age-related cause of hair loss. Around 50% of men will experience male-pattern balding by the time they turn 50, and about 80% will have it at some point in their lives. Women aren’t off the hook either—40% of women will face some form of hair thinning by the time they hit 70.[7,8]

So, what’s causing all this? It’s mostly genetic—about 80% of cases are tied to family history. The hormone behind this is Dihydrotestosterone(DHT). DHT is converted from testosterone and usually helps stimulate hair follicles. But if there’s too much of it, it can actually overstimulate those follicles. When too much DHT binds to the androgen receptors in the dermal papilla, it can cause the hair follicles to shrink, which means shorter, thinner hair over time. Eventually, if the process goes unchecked, it can cause the follicles to stop growing new hair altogether.[8]

Pattern baldness isn't just about your hair—it’s closely linked to general aging and senescence, with research showing that it may act as a marker of the aging process. Premature pattern baldness is associated with an increased susceptibility to health challenges like prostate cancer, cardiovascular disease, and diabetes.[9]

Age-Related Hair Thinning

Characterized by diffuse thinning across the scalp rather than the bald spots we often associate with pattern baldness, age-related thinning is a manifestation of the systemic aging process. Typically, around the age of 50 or 60, the anagen (growth) phase shortens, causing your hair to grow slower and shed more quickly.[10,11]

Similar to pattern balding, age-related thinning causes follicle miniaturization. What’s interesting about age-related thinning is that it’s not genetic like other forms of hair loss. Researchers are still working to understand the exact mechanisms behind age-related thinning, but it is thought that the accumulation of senescent dermal papilla cells and general systemic senescence contributes to the thinning process. As the condition progresses, fewer follicles remain in the growth phase, and the hair that does grow becomes thinner and less dense.[10]

Telogen Effluvium

Unlike other forms of hair loss that typically manifest gradually, telogen effluvium is a rapid—although reversible—form of hair loss. This type of hair loss is due to a stressful event, after which the body responds by sending a large number of hair follicles (usually 30%) into the telogen (resting) phase early. This leads to significant shedding that usually starts 2-4 months after a stressful event, which can commonly include:

• A hormonal change, including postpartum hormone declines
• Illness
• Significant weight loss and/or nutrient deficiencies

One of the most commonly affected groups is women aged 30-60 who experience rapid fluctuations in hormones through the menopausal transition and postpartum.[12]

The good news is that telogen effluvium is generally reversible and only lasts 3-6 months in most cases. Once the underlying cause is addressed, the follicles will re-enter the anagen phase, and your hair will grow back. In fact, in 95% of cases, hair loss stops within a few months after resolving the trigger. This is because the hair follicles have not stopped growing entirely; they have only entered the telogen(shedding) phase early. If the stressor continues, shedding might be prolonged—but don’t worry—using products that encourage regrowth can help speed up the recovery process.[13]

Exploring Less Common Types of Hair Loss

In addition to these more common forms of hair loss, several other lesser-known forms exist. These conditions tend to be more complex in their underlying causes and effects:

• Alopecia Areata: This type of hair loss is caused by a rare autoimmune condition where the immune system mistakenly attacks the hair follicles, leading to sudden and often patchy hair loss. Unlike gradual forms of hair loss, alopecia areata can cause quick, noticeable bald spots. It affects roughly 2% of the population, and while the exact cause is still unclear, factors such as genetics, stress, and viral infections may play a role. This often requires more intense treatments and prescription medications to treat, although many people do experience hair regrowth.[14]
• Anagen Effluvium: Anagen effluvium is primarily associated with chemotherapy or other medical treatments that target fast-growing cells. During the anagen phase, hair follicles are particularly sensitive to these treatments, leading to rapid hair loss. Unlike gradual hair thinning, anagen effluvium can result in complete hair loss in just a few weeks. Fortunately, once the underlying cause, such as chemotherapy, is removed, hair typically regrows over the next few months. Because of the damage caused by these medications, the hair may grow back differently than before with changes to its color and texture.[14,15]
• Trichotillomania: Trichotillomania is an impulse control disorder where individuals compulsively pull out their hair, leading to noticeable bald spots, particularly on the scalp, eyebrows, or eyelashes. If this continues for enough time, the hair loss can become permanent, although most cases are reversible.[14]
• Cicatricial Alopecia: This group of rare disorders, also known as scarring alopecia, causes permanent hair loss by inflaming and damaging hair follicles, replacing them with scar tissue. Although causes vary, some common sources of this condition are injury, illness, or excessive heating from hair straighteners, blow dryers, and hair extensions.[14]

Key Takeaways:

• Hair follicles are complex structures, and understanding their components—like the hair bulb, dermal papilla, and sebaceous glands—can help explain why hair loss occurs.
• Hair grows in cycles, moving through four phases: anagen (growth), catagen (transition), telogen (resting), and kenogen (potential lag phase).
• Pattern balding, the most common type of hair loss, is marked by gradual thinning in male and female patterns and is primarily genetic.
• Age-related thinning typically occurs around age 50-60, causes generalized gradual thinning instead of pattern balding, and is associated with systemic aging and senescence.
• Telogen effluvium is a temporary form of abrupt hair loss triggered by stress, hormonal changes, illness, or weight loss, usually starting 3-4 months after the trigger and lasting 3-6 months until the trigger is resolved.
• Luckily, you can take steps to slow—or even reverse—the effects of hair loss.

References

1. https://www.ncbi.nlm.nih.gov/books/NBK470321/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC3621486/
3. https://www.ncbi.nlm.nih.gov/books/NBK513312/
4. https://www.ncbi.nlm.nih.gov/books/NBK532929/
5. https://www.ncbi.nlm.nih.gov/books/NBK546248/
6. https://pubmed.ncbi.nlm.nih.gov/12218222/
7. https://www.sciencedirect.com/science/article/pii/S0306987717310411
8. https://www.ncbi.nlm.nih.gov/books/NBK278957/
9. https://karger.com/drm/article/239/4/533/836627/
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC6369639/
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC4164265/
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC3935391/
13. https://www.ncbi.nlm.nih.gov/books/NBK430848/
14. https://nyulangone.org/conditions/hair-loss/types
15. https://www.mayoclinic.org/tests-procedures/chemotherapy/in-depth/hair-loss/